Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2024 / Paper 3 / 10 / iv / Solution

A stability boundary in a mineral activity diagram must also be stable against the third mineral. Use the common-potential test for a three-mineral activity diagram. For the two corundum-producing directions,Negative values favour corundum. For the albite-to-nepheline conversion, ${\Delta }_{r}G/(RT\ln 10)=2x+11.03$, so low silica activity favours nepheline and high silica activity favours albite. These comparisons give:

All three coexist at the intersection. Pairwise mass-action equations alone do not distinguish the stable segments from their metastable extensions.